593.

Of appropriate gestures (593-600).A picture or representation of human figures, ought to be done in
such a way as that the spectator may easily recognise, by means of
their attitudes, the purpose in their minds. Thus, if you have to
represent a man of noble character in the act of speaking, let his
gestures be such as naturally accompany good words; and, in the same
way, if you wish to depict a man of a brutal nature, give him fierce
movements; as with his arms flung out towards the listener, and his
head and breast thrust forward beyond his feet, as if following the
speaker's hands. Thus it is with a deaf and dumb person who, when he
sees two men in conversation--although he is deprived of
hearing--can nevertheless understand, from the attitudes and
gestures of the speakers, the nature of their discussion. I once saw
in Florence a man who had become deaf who, when you spoke very loud
did not understand you, but if you spoke gently and without making
any sound, understood merely from the movement of the lips. Now
perhaps you will say that the lips of a man who speaks loudly do not
move like those of one speaking softly, and that if they were to
move them alike they would be alike understood. As to this argument,
I leave the decision to experiment; make a man speak to you gently
and note [the motion of] his lips.